Gas separation is important in various industries, particularly in the production of fuels, chemicals, petrochemicals and specialty products. A gas separation can be accomplished by a variety of methods that, assisted by heat, solids, or other means, generally exploits the differences in physical and/or chemical properties of the components to be separated. For example, gas separation can be achieved by partial liquefaction or by utilizing a solid adsorbent material that preferentially retains or adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the gas mixture, or by several other gas separation techniques known in the industry. One such commercially practiced gas separation process is thermal swing adsorption (“TSA”). TSA has been an important technique for purifying gases ever since Joseph Priestley separated oxygen from air using solar heat on mercuric oxide. Temperature-swing adsorption is a process wherein a bed of adsorbent is used to pull one or more species out of a stream of material, and then the adsorbent bed is regenerated (releasing the adsorbed species) by raising the temperature of the bed.
TSA has the advantage that by swinging the gas mixture's temperature, instead of the pressure, compression costs can be avoided. Another advantage of TSA is that adsorption isotherms are strongly influenced by temperature. Thus, very high purity products can be obtained by adsorbing impurities at low temperature (where adsorption is strong) with the release of a strongly held impurity species being possible by means of high temperature for desorption. However, TSA has several disadvantages. For example, the time to swing adsorbent beds over a temperature range sufficient to affect the separation can be relatively long, which means the equipment must be very large and therefore economically unattractive. Also, heat integration of the TSA cycle, upsets of downstream equipment, and the dilution of product by a large amount of gas used to raise the temperature of the bed are additional disadvantages of TSA processes.
Various methods of supplying heat to the adsorbent for regeneration have been proposed. These include microwave energy (U.S. Pat. No. 4,312,641), installation of electrical heaters inside the packed adsorbent bed of the adsorber (U.S. Pat. No. 4,269,611) and direct application of electric current to the adsorber for electrodesorption (U.S. Pat. No. 4,094,652). U.S. Pat. No. 5,669,962 discloses a dryer comprised of a shell and tube type adsorber heat exchangers wherein the internal tube surface is coated with fine water adsorbent particles. The dryer can be used in rapid thermal swing cycle process. The adsorbent is indirectly heated or cooled by flowing hot or cold feed gas to the separation process through the shell side passage of the heat exchanger. The feed gas acts first as a cold shell side gas in a first absorber heat exchanger then is heated to act as a hot shell side gas in a second absorber heat exchanger undergoing regeneration, and then passes through the tube side of the first absorber heat exchanger where it is dried. Part of the dried gas is used as a purge gas for the tube side of the second absorber heat exchanger. Interchanging the functions of the two adsorber heat exchangers periodically reverses the cycle. The interchange may take place at intervals of from thirty seconds to three minutes. Many of the TSA processes have cycle times significantly longer than this, often as long as 12 hours.
Several approaches have been taken to overcome one or more of the above mentioned disadvantages. For example, one approach was to integrate a heat exchanger with a sorbent material. U.S. Published Patent Application No. US2003/0037672A1 discloses a rapid thermal swing adsorption process wherein separation of contaminants, such as water, from a gas stream such as air is performed using adsorbent packed in tube side passages of a tube and shell heat exchanger adsorber. After a period of adsorption heating fluid is passed through the shell side passage of the adsorber during regeneration and upon exiting from the adsorber is recycled via a heater back into the shell side of the adsorber. During a cooling phase of the regeneration, a cooling fluid is passed through the shell side passage of the adsorber.
U.S. Pat. No. 6,293,998 teaches a spirally wound module, for pressure and temperature swing adsorption processes. The spirally wound module provides high efficiency gas separations by reducing the differential pressure required between the adsorption pressure and the desorption pressure. The apparatus comprises an adsorption zone containing at least one adsorbent paper layer containing a selective adsorbent and an adsorbent spacer spirally wound about a hollow mandrel and in intimate thermal contact with a heat transfer zone.
While attempts have been made in the TSA art to provide a process without the disadvantages previously mentioned none have succeeded in developing a TSA process that is robust enough to make TSA more commercially viable than conventional TSA processes. Therefore, there still remains a need in the art for improvements to the TSA process that can overcome some of the economical and technical hurdles of conventional technology.